Cans for use as containers for food or drink are often formed from two parts, a cup and a can end. The cup at least is often formed from tin-plated steel, the tin coating providing corrosion resistance. The cups are usually made by drawing and ironing so that the wall especially is stretched considerably during the forming step. To reduce the cost of the raw materials it is desirable to use steel carrying a coating of tin which is as thin as possible. During the forming step the stretching of the thin tin coating often reveals areas of steel surface. These areas must be protected from corrosion.
It is known to coat the metal surface with a lacquer. The surface may be pretreated with an alkali degreaser/cleaner to improve adhesion of the lacquer, but the treatment by the alkali does not in itself improve the corrosion resistance.
It is known to provide a light iron phosphate coating on the insides of cans followed by a coating of lacquer. The conventional phosphating solutions do not coat tin metal so that conversion coating a surface having areas of iron and tin metals gives a surface having areas of tin metal and of iron phosphate. Although this may provide a satisfactory base for a subsequent lacquer coating, the discontinuities in the surface may show up undesirably, especially if the lacquer is transparent.
In GB Nos. 2033432 and 2068418 it is proposed to conversion coat tin-plated steel using a phosphating solution containing tin phosphate.
In GB No. 2033432 a conversion coating solution contains phosphate and stannous ions and a large excess of fluoride. The pH of the solution is in the range 5.5-6.5. In GB No. 2068418 a conversion coating solution contains phosphate and stannous ions, chlorate and/or bromate as accelerator and chloride ions and has a pH in the range 3.5-5.3 although it is stated the pH may be as low as 3. The processes in the two specifications do not however give satisfactory results. Often they are no better than using an alkali cleaner alone. The coating formed on the tin surfaces is often so thin as to be undetectable. The coating does not provide a satisfactory base for a subsequent lacquer coating nor does it provide a significant improvement in corrosion resistance.